I'm not in exactly the same position but I do have a pair of 11s that are pretty old now. I've been thinking of buying some 12s but after reading your post I'm not so sure. I certainly don't want anything heavier or bulkier. Maybe another pair of 11s is the answer..?
I had a disappointing run at London this year and have also found it harder to recover from than previously. As far as I know I didn't have a bug. Perhaps it's got something to do with the amount of effort needed if you're not having a good day. Whereas if everything's coming together nicely, maybe that's less stressful on your body (and your mind).